1. Field of the Invention The invention is related generally to the field of electromagnetic induction resistivity well logging instruments. More specifically, the invention is related to a method and apparatus for calibration and pre- and post logging verification for an induction resistivity well logging tool operating in the frequency and/or time domain for reduced errors introduced into the acquired logging data.
2. Description of the Related Art
Electromagnetic induction resistivity well logging instruments are well known in the art. Electromagnetic induction resistivity well logging instruments are used to determine the electrical conductivity, and its converse, resistivity, of earth formations penetrated by a borehole. Formation conductivity has been determined based on results of measuring the magnetic field of eddy currents that the instrument induces in the formation adjoining the borehole. The electrical conductivity is used for, among other reasons, inferring the fluid content of the earth formations. Typically, lower conductivity (higher resistivity) is associated with hydrocarbon-bearing earth formations. The physical principles of electromagnetic induction well logging are well described, for example, in, J. H. Moran and K. S. Kunz, Basic Theory of Induction Logging and Application to Study of Two-Coil Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of Exploration Geophysicists, December 1962. Many improvements and modifications to electromagnetic induction resistivity instruments described in the Moran and Kunz reference, supra, have been devised, some of which are described, for example, in U.S. Pat. No. 4,837,517 issued to Barber, in U.S. Pat. No. 5,157,605 issued to Chandler et al and in U.S. Pat. No. 5,600,246 issued to Fanini et al.
The conventional geophysical induction resistivity well logging tool is a probe suitable for lowering into the borehole and it comprises a sensor section containing a transmitter and receiver and other, primarily electrical, equipment for measuring data to infer the physical parameters that characterize the formation. The sensor section, or mandrel, comprises induction transmitters and receivers positioned along the instrument axis, arranged in the order according to particular instrument or tool specifications and oriented parallel with the borehole axis. The electrical equipment generates an electrical voltage to be further applied to a transmitter induction coil, conditions signals coming from receiver induction coils, processes the acquired information, stores or by means of telemetry sends the data to the earth surface through a wire line cable used to lower the tool into the borehole.
There are a few hardware margins and software limitations that impact a conventional transversal induction logging tool performance and result in errors appearing in the acquired data.
The general hardware problem is typically associated with an unavoidable electrical field that is irradiated by the tool induction transmitter simultaneously with the desirable magnetic field, and it happens in agreement with Maxwell's equations for the time varying field. The transmitter electrical field interacts with remaining modules of the induction logging tool and with the formation; however, this interaction does not produce any useful information. Indeed, due to the always-existing possibility for this field to be coupled directly into the receiver part of the sensor section through parasitic displacement currents, it introduces the noise. When this coupling occurs, the electrical field develops undesirable electrical potentials at the input of the receiver signal conditioning, primarily across the induction coil receiver, and this voltage becomes an additive noise component to the signal of interest introducing a systematic error to the measurements. Fortunately, there are proven techniques allowing for suppressing of these undesirable effects that were discussed in the above-mentioned U.S. Pat. No. 6,586,939 B1.
Further improvements of induction measurements have been associated with long-time tool reliability and, in particular, with overall stability of transmitter-receiver magnetic moments as well as accounting for possible gain and phase changes while operating in wide range of well bore temperatures.
Thus, for those who are skilled in the art it could be understood that operating downhole instruments in rapidly changed environmental conditions results in changes of the tool geometries including elongation of the induction coils and even their micro-dislocation along instrument mandrel. These effects exist due to different and finite temperature expansion coefficients of the materials the tool mandrel was built from, some imperfections in tool manufacturing processes, etc. Typically, the design of downhole instrumentation targets hardware solutions where these changes are monotonic, preferably linear with the temperature and have no hysteresis, i.e., are fully reversible. Apparently, if these changes are not accounted while the acquired data is processed, it leads to significant errors in calculation of final formation resistivity values.
To enable correct processing of the acquired data the downhole tools should pass through comprehensive temperature, pressure and vibration testing during their manufacturing. The results of these tests typically form correction tables, usually unique for each instrument, that are either stored in the tool on-board firmware or in the surface processing unit.
However, having these tables is not always sufficient as it's almost impossible to predict every condition the tool could be in during transportation and operation. An ability to quickly verify the instrument parameters on-site, run fast pre- and post-logging verifications and be able to check them during measurements even while logging would further enhance measurement accuracy.
Thus, there is a need for a method and apparatus to conduct these performance verifications or calibrations so that they can be processed and accounted for in acquired data.